The present invention relates to an optical encoder for detecting a position of an object to be detected moving along a straight line or a rotation angle of a rotating member.
FIG. 8 is a diagram showing an arrangement of a conventional optical fiber type encoder. In this figure, a reference numeral 801 depicts a light source, 802 and 807 respectively depict a transmission side optical fiber and a receiving side optical fiber, 803 and 806 respectively depict a transmission side collimator lens and a receiving side collimator lens, 804 a stationary slit plate, and 805 a rotating slit plate attached to a rotary shaft which is a subject to be detected. Further, a reference numeral 808 depicts a photo detector.
Then, the explanation will be made as to the operation of such a conventional optical fiber type encoder. A light beam irradiated from the light source 801 passes through the transmission side optical fiber 802 and converted in parallel spatial light by the transmission side collimator lens 803. The spatial light irradiated from the collimator lens is illuminated on a slit formed on the stationary slit plate 804. The slits of the rotating slit plate 805 are arranged with such a pitch that the slits divide the outer periphery of the rotating slit plate with an equal interval so as to intermittently transmit the light therethrough. The stationary slit plate 804 is also provided with slits having the same configuration as those of the rotating slit plate 805 only at the light-illuminated area thereof. According to such an arrangement, the light transmitted through the stationary slit plate 804 is provided with light intensity distribution according to the configuration of the slit thereof and then illuminated on the rotating slit plate 805. The rotating slit plate 805 is also provided with the slits of the same configuration as that of the stationary slit plate. When the rotating slit plate is rotating, a light quantity of the light passed through the rotating slit plate 805 becomes maximum when the slit of the rotating slit plate is located at the rotation position coinciding with the bright portion of the light passed through the stationary slit plate 804, but becomes minimum when coinciding with the dark portion of the stationary slit plate. The light passed through the rotating slit plate 805 is coupled with the receiving side optical fiber 807 through the receiving side collimator lens 806 and then transmitted to the light detector 808, which in turn converts the light into an electric signal and outputs it. In this manner, since the light transmitted to the photo detector 808 becomes a signal which blinks repeatedly in accordance with the rotation of the rotating slit plate 805, the rotational position of the rotating slit plate 805 can be detected by counting the number of the blinking. However, the rotation direction of the rotating slit plate can not be detected from the single blinking signal. In other words, in order to detect the rotation direction of the rotating slit plate, it is required to provide another optical system which outputs a signal whose phase differs by 90.degree. from the signal output from the photo detector. Further, it is required to provide still another system for correcting the initial position of the rotating slit plate which outputs a pulse at every revolution of the rotating slit plate. Thus, the conventional optical fiber type encoder requires three optical systems in total. The aforesaid encoder is an incremental type encoder which counts the increase or decrease of the blinking of the light. Such an encoder requires, in order to detect the absolute position of the rotation of the rotating slit plate, a plurality of slit tracks arranged to deliver a binary blinking signal in accordance with the rotational position of the rotating slit plate and the same number of optical systems for detecting the blinking of the signal.
Since the conventional optical fiber type encoder is arranged in this manner, there is a problem that the arrangement of the encoder is complicated, the manufacturing thereof is difficult and the cost of the encoder is high since a plurality of optical fibers are required for signal transmission.